Be Excellent To Each Other

You've got a square with sides S long. I don't know S, and it's likely going to change periodically.
The top-left grid reference is -180, 85.
The centre is 0, 0.
The bottom-right is 180, -85;

I know X and Y, and if X is 0 it needs to map to the far left, and if X = S then it needs to map to the far right.
The same for Y, 0 = top and S = bottom.

So. What calculations do I have to perform on X and Y to get them to map properly? My brain is tired and now I'm hungry, so I'll see if someone has an answer when I get back with my lunch.

(It's probably not hard to guess which of my other threads today relate to this)

You're going to do your own Street View program?

No answers? I expected the geeks to be slobbering over this one.

I didn't want to type it out then find Mr Dave had already done it.

Why do you say S can change, then give us the numbers?

Assuming you have 'S' as a variable?

MapWidth = 360
MapHeight = 170

SWidth = 100
SHeight = 100

Sx = 50
Sy = 50

Mapx = ((MapWidth/SWidth)*Sx)-(MapWidth/2)
Mapy = ((MapHeight/SHeight )*Sy)-(MapHeight/2)

does that seem right?

-180 + (X/S)*180*2

and

85 - (Y/S)*85*2.

I think.

Thanks, all!
I have used:
x = ((x-(s/2))/s)*360;
y = (((y-(s/2))/s)*170)*-1;

Alas, I have only just arrived, and am too late.

How frequently would you use them, as that could be fairly easily optimised. For a start z*170*-1 = z*-170.

360/s, -170/s and s/2 could also be precalculated and stored, avoiding most of those division ops division ops frequently. Or indeed I'd imagine you can take the reciprocal, and avoid them altogether.

Only about (max) 30 times per page load, so optimisation isn't really an issue.

I would expect my compiler to take care of such basic arithmatic re-ordering optimisations like that.

/kicks Javascript

It still gets compiled and it still gets optimised. Where do you think all these massive 10x-20x-30x speedups are coming from in modern browsers?

Migrant workers?

Only in certain circumstances though - And that isn't just reordering, it relies on s being constant, not something your compiler would always assume, it's somewhat context dependant, the kind of thing aliasing would play havoc with.

I find the answer is usually to start crying. Hope this helps!

Fuck, I can't think today!
Right, same thing, except now I've got he answer (somewhere between -180:85 and 180:-85) and I want X and Y in regards to S (which I still know). So basically, the same thing again, but backwards.
Why is my brain so dumb?

Yes. It helped me laugh this evening.

x'= ((x+180)/360)S
y'= ((y+85)/170)S

(obv you can expand brackets to prevent mr dave upset)